1. Field of the Invention
This invention relates to a columnar solid body rotor, and more particularly to a columnar solid body rotor for DC dynamo electric machines in which a closely stacked winding is mounted on the outer periphery of a dust core.
2. Description of the Prior Art
A method that has heretofore been employed for increasing the output of a DC motor, in particular, a DC servo motor, and for enhancement of its control function is to increase the current density of the rotor winding to raise the upper limit of temperature, or to effect forced cooling. If, however, it is possible to produce a rotor which is small in inertia, extremely small in the loss of the speed function and mechanically rigid, the motor output can be increased by a high-speed rotation.
Incidentally, as the iron loss, especially the eddy-current loss, which is attendant with the high-speed rotation, increases in proportion to the square of a rotating magnetization frequency, a rotor using a laminated iron core of silicon iron, a magnetic alloy or the like suffers excessive iron losses, and accordingly it is difficult to increase the output by the high-speed rotation. On the other hand, a coreless rotor is free from iron losses, so that an increase in the output by the high-speed rotation is possible.
Coreless rotors heretofore employed in DC motors may be of two types, one of which is a disc-shaped rotor formed with a disc-shaped printed circuit or a punched copper plate or a winding formed into a disc-shape, whereas the other is a cup-shaped rotor arranged such that a single-layer or multi-layer winding is formed into a cup-shape.
However, the disc-shaped rotor formed with the printed circuit or punched copper plate has a small torque factor and a relatively large moment of inertia owing to its construction. Since the rotor of the type that a winding is formed into a disc-shape is large in the moment of inertia and, by nature, low in the torque-inertia ratio, its kinetic energy due to a high-speed rotation is very large, which leads to the generation of distortion and vibration. Therefore, any of the disc-shaped rotors as mentioned above are not suitable for the high-speed operation. On the other hand, the cup-shaped rotor has a limit in its field flux density, so that in the case where it is designed for a high output, the volume of the winding is increased which inevitably increases the diameter and thickness of the cup structure to cause an increase in the moment of inertia of the rotor. As a consequence, the torque-inertia ratio of the rotor tends to decrease with an increase in its output rating.
Further, the rotor shaft and the cup-shaped winding are mechanically coupled with each other in a cantilever fashion such that the cup-shaped winding is affixed at its bottom, i.e. at one end in its axial direction, to the rotor shaft. Accordingly, in the case of high-speed rotation or when applied an impulsive input, the cup-shaped winding and its fixed portion are physically distorted or deformed which may lead to breaddown of the rotor itself in some cases.
In view of the above, the present inventor has previously proposed a columnar solid body coreless rotor whose volume is almost occupied by a winding and a stator for a DC dynamo electric machine in which intense magnetic field poles are disposed on the outside of the rotor to provide a highly efficient magnetic field path which blocks an excessive leakage flux inevitably occurring in a magnetic field path of a large air gap length.
The abovesaid columnar solid body coreless rotor is such as shown in FIG. 1 in which a sleeve-like insulator 16 is disposed around a rotor shaft 11 and a closely wound rotor winding 15 is mounted on the outer periphery of the insulator 16. Reference numeral 14 indicates spacers. This rotor is a rod-like one whose volume is almost occupied by the winding. Accordingly, the volume of the winding is large, which offers the advantages that the rotor is low in inductance, mechanically very rigid and small in inertia. Moreover, the rotor exhibits excellent control performance and is excellent in commutation because of its low inductance property, and in addition, since the rotor is small in inertia and very rigid mechanically, it has the performance that withstands a high-speed rotation of 20,000 to 40,000 rpm, for instance, and an impulsive input. The performance of the rotor will hereinbelow be described.
The torque which is produced by the rotor of a DC dynamo electric machine is as follows: Let l, r, B and i represent the length [cm] of each conductor in its axial direction, the radius [cm] of the conductor at its center, the flux density [gauss] of an air gap and a conductor current [A], respectively, and let it be assumed that the number of poles is two and that the number of parallel circuits in the rotor is two. In the following the cgs unit will be used in consideration of measured values described later on. The force f generated by the conductor is that f=Bl(i/10)[dyne], and accordingly the turning force .tau. is that .tau.=fr=Bl(i/10)r[dyne-cm]. Letting Z represent the total number of conductors, the number of conductors included in an angle d.theta. is ##EQU1## and the turning force acting on this portion is ##EQU2## Accordingly, the total turning force is given as follows: ##EQU3## Blrd.theta. corresponds to the number of positive and negative fluxes of each pole, that is, twice of the flux .PHI. of the rotor in this case, so that if a rotor current is taken as Ia, it follows that ##EQU4## Accordingly, equation (1) becomes as follows: ##EQU5## Thus, the torque generated by the rotor is obtained which is given by equation (2). If the number of revolutions of the rotor per second is taken as n[rps], the power generated by the rotor, that is, its output Po is expressed by the following equation: EQU Po=2.pi.nT.times.10.sup.-7 [W] (3).
Substituting T of equation (2) in equation (3), it follows that EQU Po=nZ.PHI.Ia.times.10.sup.-8 [W] (4).
Since the output by equation (4) is obtained by substituting the generated torque of equation (2), it includes a loss torque Tl which is the sum total of mechanical losses by bearings and a brush and wind losses and, in the case of a core being used, iron loss; namely, (T-Tl) is the axis transmission torque. Accordingly, a true output Pe is as follows: EQU Pe=2.pi.n(T-Tl).times.10.sup.-7 [W] (5).
It appears from the above that, according to equation (4), the output of the abovesaid columnar solid body coreless rotor proposed previously by the present inventor is in proportion to the rotor effective flux .PHI., the rotor speed n and the rotor current Ia.
Since the product of the conductor number Z and the rotor current Ia depends upon the volume of the rotor winding and the upper limit of a permissible temperature rise of the winding, the rotor output can be enhanced by increasing the effective flux .PHI. and the speed n of the rotor. It is also evident from the general formula of the starting time constant that an increase in the effective flux of the rotor provides for enhanced control performance.
Further, as the columnar solid body coreless rotor is capable of high-speed rotation which is unobtainable with the conventional coreless rotors as mentioned previously, the speed n has already reached substantially a desired value. If the effective flux of the rotor can be increased without a sacrifice of the requirements for the high-speed rotation, it is possible to provide for enhanced output and control performance of the rotor. However, if the flux density of the magnetic field poles disposed on the outside of the columnar solid body coreless rotor is raised twice, the leakage flux of the magnetic field path also increases with an increase in the flux density between the poles in the case of a magnetic field by permanent magnets, so that a required length of each permanent magnet in a closed curve of the magnetic field path becomes twice or more the cross-sectional area of the magnet also increases. As a consequence, the configuration of the magnetic field path becomes several times larger to degrade the configuration-output ratio, and an increase in the number of permanent magnets used leads to raised manufacturing cost, which causes a bottleneck in wide application of the rotor.
However, since the columnar solid body coreless rotor is non-magnetic, its space permeability is that .mu..sub.0 =1. If the volume-conversion permeability (the permeability of the rotor as a whole) is increased by a desired factor and if the iron loss, especially the eddy-current loss can be made negligibly small in its ratio to the rotor output, it is possible to reconcile an increase of the rotor effective flux .PHI. and a rise of the rotor speed n.
Of iron losses of a magnetic substance in the case of the rotating magnetization changing at high speed, the eddy-current loss presents a problem. For example, in the case where magnetization is changed along the longer axis of a columnar magnetic piece of a radius r.sub.0, letting J, .rho. and k.sub.1 repsesent magnetization, the resistivity of the magnetic material and a constant, respectively, the eddy-current loss Pd per unit volume can be approximately expressed by the following equation: ##EQU6## That is, the eddy-current loss is in proportion to the square of the magnetization changing speed, so that, in the case of a revolving member, the eddy-current loss rapidly increases with a change of the rotating magnetization, i.e. an increase in the revolving speed. Moreover, it is seen that the eddy-current loss is proportional to r.sub.0.sup.2 and hence can be markedly decreased by making fine the magnetic material used.
To make the magnetic material fine can be achieved by solidifying a finely powdered magnetic material and an insulating material such, for example, as a synthetic resin into a dust core. In other words, the dust core has an insulator with permeability .mu..sub.0 =1 interposed between particles of the finely powdered magnetic material, and as a consequence, the effective magnetic field is markedly reduced under the action of the reverse magnetic field by the influence of the magnetic pole of each particle, thereby to cause a sharp decrease in the eddy-current loss and, in addition enable the use of the dust core at high flux density.